Investigating the self-assembly of pH-sensitive switchable diamine surfactant using sum frequency generation spectroscopy and molecular dynamics simulations

The responses of the N-alkyl diamine groups to variations in pH affect their conformations and surface activities, making them relevant to applications relying on interfacial interactions, such as controlled emulsification and mineral flotation. An in-depth understanding of interfacial self-assembly is crucial. Herein, a molecular-level study was performed to investigate the adsorption and self-assembly of N-dodecylpropane-1,3-diamine (DPDA) at the air-water (A/W) interface using sum frequency generation (SFG) spectroscopy and molecular dynamics (MD) simulations. The SFG spectra of DPDA, acquired under three pH conditions, suggest that the protonation of the DPDA diamine group influences the alkyl chain arrangement at a varying degree at the A/W interface. Analysis of the di-cationic DPDA SFG spectrum at a low pH showed fewer gauche defects at low concentration, as indicated by the relatively higher intensity ratio (ICH3SS/ICH2SS) of 18.1 ± 0.6. The density profiles from MD simulations at different surface areas per molecule and pH conditions, showing varying degrees of packing, support the observation of gauche defects in SFG. With MD simulation, the radial distribution factor for di-cationic species has the highest probability of forming hydrogen bonds compared to mono-cationic and non-ionic species. These g(r) probability results conform with observations obtained from SFG spectroscopy, where we observed a strong hydrogen bond interaction at low pH conditions with di-cationic species, forming tetrahedrally arranged water molecules at the A/W interface. Overall, comprehensive insights will facilitate the visualization of alkyl diamines and their potential derivatives at the A/W interface, enabling a better understanding of their behavior across various applications.